Process of producing stainless steel sheet or strip stock



Patented Feb. 1944 raocrss F PRODUCING STAINLESS smar su ar on s'rmr srocn Gordon Gage and David Hughes, Butler, Pa, as: A signers to The American Rollin: Mill Company, Middletown, Ohio, a corporation of Ohio No Drawing. Application June 15, 19M, Serial No; 398,650

12 claim (01. 148-12) v General objects of our invention are to provide a process for the production of stainless steel strip or sheet stock which increases the yield of the material, which provides'a better surface on the sheets, whichsaves power. which increases the life of rolling mill rolls and keeps them bright,

- clean and shiny, which permits the production of a greater tonnage on given apparatus and which I therefore saves time and money per ton, and

which eliminates certain picklings and annealmgs.

These general objects will be better understood from the description which-follows. We accomplish them, and others which will be pointed out hereinafter, or will be apparentto the skilled rolling millman, by certain variations in the usual procedure which we shall now describe by contrast with current practices.

Ingots of stainless steel usually vary in weight from say 5% tons down to 3 tons, averaging about 8500 pounds. The average size is l'7x42x66 inches. Thecomposition oi the ingots varies considerably. Someof them may comprise 18% of chromium, 8% of nickel, balance substantially all iron, others may have around 17% of chromium, with less nickel or none at all, balance being substantially ali iron, or either of these exemplary analyses may additionally contain molybdenum or columbium or other materials. The size 01 the annealed ingot may be taken into account in variations of the hot rolling procedure in known manner. As to the analysis of thematerial, we

have found that our process is applicable to all analyses of materials coming under the commercial classification of stainless steels.

In the accepted practice the stainless steel in- 7 gots are soaked in the usual manner, being thereby heated to rolling temperature. Then they are sent through a blooming mill. That is to say, they are rolled back and forth one universal mill to.

produce a bloom which; by way of example, may

be from approximately 17 inches down to 12% inches in thicmess.

The bloom is reheated and is then rolled down to a slab, say 2 to 4 inches in thicmess, 18

inches wide and 37 inches long, or any other suitable slab size depending upon the desired final gage and dimensions.

. The slab is then permitted to cool and prior to any further rolling operations is usually thoroughly cleaned. This may be done by grinding its surfaces with swing grinders so as to erase all scabs, scale and the like.

Alter reheating the cleaned slab; it is the usual practice to reduce it'by hot rolling on a universal mill to from inch to k inch in thickness, dea pending upon the hot mills requirements for the finished gage. From 5 to '15 forward and reverse passes through. the universal hot mill are usually iven, depending upon the grade, the exigencies of subsequent reduction and the like. In our owh commercial practice the material is then .passed on down a table to'a series of tour-high hot finishing tandem mills and is there reduced to a gagefor cold rolling. which normally will be between .080 inch and a quarter inch, depending on the order. Thetandemmill arrangement produces a strip which is then coiled.

The coiled strips are subsequently decoiled, welded together into a continuous strip. and then passed through a continuous annealing furnace.

A pickling, washing, scrubbing and drying is next practiced upon the coils and, when they are recoiled, they are ready forcold reduction.

While we have thus far described a current and exemplary hot rolling process for producing intermediate gage material suitable for subsequent cold rolling, it will be understood that variations in the procedures are to be found both in the current art and in our novel process as hereinafter set forth. We do not modify the hot rolling procedures except in certain instances as to the hot rolled gage produced, all as we shall more fully explain hereinafter.

The cold reduction of sta ess steel volves certain serious pro ems. Heretofore' it has been impossible successfully to cold roll stain less steels with more than a 40 to reduction between annealings. Even with the best prior practices, a 50% reduction between annealings is all it is practical to make with stainless steel. From this point any further screwing down of the mill merely manifests itself in additional mill housing stretch or further deformation of the mill rolls. No greater general reduction in gage is produced; but localized reductions may impair the quality'of the work piece. As a consequence, it is the usual practice, having brought the materiaito the hot gages above noted, to carry it own to finished thickness by a long series of cold rol1-.

ing reductions with intermediate annealing and pickling treatments. With the usual cold reductions of say 40% in good practice for the first operations and somewhat lessened reductions for subsequent operations, from 4 to 6 or more stages have heretofore been found necessary. For example, in rolling stock which leaves the hot mill at the fairly light gage of .140 inch to a finish gage on .018 inch, it is standard practice to have stocks inas many as five anneallngs and picklings between the cold reductions.

We have found that by certain procedures and treatments we can reduce stainless steels without reannealings and repicklings by very much greater percentages of reduction, so that the number of heat treatments and cleaning steps is very greatly cut down. We accomplish this by producing on the surface of the sheets or strips a particular coating in a heat treatment, which coating enables us to make very much heavier cold rolling reductions. We do not wish to be bound by theory; but we believe that our coating acts as a lubricant in the cold rolling operation. It permits us to do more cold rolling on a hard sheet; and we do not get any "pick up? on the strip from the rolls while the strip is being rolled. In the old practices the scum on the rolls,

which causes the rolls to grip the strip,'prevents the metal from flowing, we believe, and therefore prevents further reduction in gage after approximately 50% reduction in the best practices. our process the metal never stops flowing.

In an exemplary procedure under our invention, we handle the ingots, blooms and slabs in the usual manner and produce by hot rolling in usual way a strip or strip sheet. We find it advantageous, and good mill practice, to get as light a hot rolledgage as we can, dependent upon the required finished gage of the material. By way of illustration, let it be supposed that we have produced on the hot mills a material of a sage of .140 inch. We anneal and pickle this material in the usual way and then take it down to .070 inch on atandem cold mill. This is a 50% reductlon and is in line with the first cold rollin of the standard practices. It will be understood j pickle the strip after this annealing.

The purpose of the annealing is to, produce on the strip an extremely thin, controlled him of oxide. This may be done in various ways, and is normally done by us through control of the fur-v nace atmosphere so as to produce such a coating. The oxide coating on the strip should be less than 5 0 inch thick. it appears greenish in color and probably consists of or includes a chrome oxideor a chrome-nickeloxide, It is uniform over all surfaces of the stripor sheet,;

and is of such thinness that it is possible to see the cold rolled color of the stainless steel shining through it. The surface may be described as having a green-yellowish cast, but generally a metallic and shining appearance.

Such a coating may be made by passing n smp through a gas fired furnace and heating it to approximately 800 F.; and our invention ineludes such treatments. We prefer, however, to employ a more exact control by the use of special atmospheres in the furnace. The atmosphere which we prefer to employ is almost pure nitrogen, about 4% only of the gas being combustible.

Other gases, however, may be employed. An annealing in hydrogen is possible; but we have found that nitrogen gives a brighter strip and apparently a somewhat more adherent oxide coating. Also mixtures of nitrogen and hydrogen with each other or with other gases arefurnace is composed of four sections. The tome perature in the first two sections is maintained between 1900 and 1950 and the temperature in the last two sections is abont 1950 or slightly higher. The strip upon leaving the last of these sections passes through two enclosed cooling hoods about 16 feet in length and then into a water cooled cooling section approximately 11 feet long. While still protected by the special furnace atmosphere it is cooled to around 600 F. before it is brought out into the air. It will be understood that the dimensions of the furnace and its construction, as well as the speed. of travel of the piece, and other factors, may, like the nature of the atmosphere, be considerably varied by the skilled worker in the art to obtain the desired result. The desired result is the formation of the thin, uniform, controlled and tightly adherent oxide film which we have described above.

Having produced this film in an annealing treatment, as explained above, we do not then pickle the strip, but we send it without further treatment to the cold rolls where it is further reduced. In our particular example the material which is given the oxide treatment is at a gage of .070. This material as so treated can be carried down by cold rolling without any further annealing treatments to as thin a gage as .015; Similarly with a. stock measur- 3 inch or thinner. ing .050 inch in thickness, and given our treatment, we can cold roll to .006 inch without intermediate annealing. With a treated material hav- This'gives approximately 2% hydrogen'and approximately 2% CO. In our commercial practice we continuously recondltion the gas in our furthrough a'ctivatedaalumina.

.nace by recirculating it. In the recirculation wcycle carbon dioxide and moisture are removed in any suitable way, as by passing the gas .005 inch without any difliculty and without in I termediate annealing.

In this way it will be seen that, depending on the hot mill gage, we can go down to very light gages with only one intermediate annealing and with no intermediate pickling, at a very great saving in processing cost. Where it is impracticable to finish on the hot mill at lighter than say .140 inch, but where the finished gage is required to be as light as .005 inch, our procedure is to reduce on the cold mill as described to .070 inch, give the material our special annealing treatment, cold reduce it to say .030 inch, then pickle it, then give it another special annealing treatment without subsequent pickling, and finally reduce it to .005 inch without further'heat treatments. In such a process we have two annealings and one pickling step in contrast to the 7 or 8 anneals and pickles which would ordinarily be required. Also'it is possible in many instances to reanneal without pickling.

- The final finishing steps, of course, both in the old procedure and in our new procedure, comprise a final anneal for softness, and then a temper rollings. This is because the material when it isgivenour special anneal, should be characterized by a cold rolled rather than by a hot rolled surface. The best results are not secured,we find, by merely pickling the scale from a hot rolled strip or sheet and then giving it our special anneal preparatory to any cold rolling. It is possible, of course, to take hot rolled strip, thoroughly clean it by pickling, sand blasting .or the like, and then give it a cold rolled surface by passing it one or more times through cold rolls without producing any very'great reduction.

Then it may be'given our so called special anneal. In ordinary procedures, however, this would not produce as much saving as the routings described above; but it has its uses, especially in producing very light gages, and is not outside the scope of our. invention.

The thin oxide, film, produced in our process -need not be pickled away in instances where pickling would not otherwise be necessary. The

I or 8, without first removing the film.

Modifications may be made in our invention reduction in excess of substantially 50%, while the surfaces of the metal are still covered with I the said coating.

2. A process of producing stainless steel sheet or strip which comprises hot rolling the metal to intermediate gauge, pickling the metal, thereafter cold rolling it at least sufliciently to impart to its surfaces a cold rolled finish, then by a heat treatment, coating the surfaces of said metal with a uniform thin coating of oxide, greenish incolor and not thick enough to obscure the cold rolled sheen of said surfaces, and without pickling, further cold rolling the metal.

3. A process as claimed in claim 2 in which the first cold rolling is a substantial reduction of the gauge of the metal but not exceeding 50%, in

an intermediate gauge, annealing and pickling the metal, then reducing it substantially 50% by a cold rolling, then annealing the metal so as to produce on its surfaces a controlled film of green- 5 ishoxide uniform in character and of such thinhas as not to obscure the cold rolled sheen of the metal, and thereafter without pickling,

reduction of more than substantially 50%. lo 5. The process as claimed in claim 4 in which the gauge of the material after hot rolling is of the order of .140 inch, after the fir'stcold rolling is of the order of .070 inch, and after the second cold rolling is of the order of .015 inch;

6. The process as claimed in claim 4 in which the gauge of.the material after hot rolling is of the order of' .140 inch, after the first cold rolling is of the order of .070 inch, and after the second cold rolling is of the order of .015 inch, and finally annealing and pickling the sheet or stripthus formed;

7. A process as set forth in claim 1 in' which the heat treatment is an annealing in a controlled atmosphere.

the heat treatment is an annealing in an atmosphere of substantially Pure nitrogen.

9. The process as set forth in claim l-in which the heat treatmentuis an anneallng'in an atmosphere of substantially pure hydrogen.

10. Aprocess as claimed in claim 1 in which the heat treatment is a continuous annealing at a temperature of the order of 1900 to' 1950 F. in a controlled? atmosphere.

11. A process of producing stainlessfsteel sheet or strip which comprises hot rollingthe metal to an intermediate gauge, annealing, and pickling the metal, cold rollingit with approximately 50% reduction, heat treating the metal so as 40 to form a uniform coating of greenish oxide on its surfaces of sufiicient gto expose the cold rolled sheen of the metal, then without pickling, further cold rolling-the material with a reduction of greaterthanfsubstantially then pickling the material and again annealing it was to produce upon its surfacesthe same thin, uniform and controlledcoatingof oxide,-

and then again cold rolling thematerial with a reduction greater than substantially 50%. i

12. A process of producing stainless steel sheet or strip comprising hot. rolling the metal to an intermediate gauge, then annealing, pickling and cold rollingit to substantially the limit of prac ticable cold rollingreduction', then preparing the metal for further cold rolling reduction beyond said limit by annealing it under conditions to produce upon its surfaces a coating of oxide of uniform character and of the order of thickness of substantially M inch, and thereafter cold reduction greater 60 rolling the -material with a than substantially 50%.

' i GORDON GAGE.

DAVID HUGHES. 3

cold rolling the material to a final gauge with a 8. The processes set forth in claim 1 in which 

